Silicon semiconductors play a key role in most types of solar energy conversion due to their electro-optical properties and the technology of production of such cells is highly developed. It is one of the serious drawbacks of such cells that they are quite expensive and thus their widespread use for the conversion of solar energy to electrical energy is not yet economical. One of the ways to overcome this drawback is to concentrate the incoming flux of solar radiation onto a solar cell, increasing conversion efficiency and substantially reducing the surface area of the solar cells required per unit area of solar flux. Various suggestions have been made how to focus solar energy on solar cells, but conventional means concentrate the entire solar radiation, including heat radiation. Solar cells generally have a maximum of sensitivity about 800 nm, and if heat energy and radiation in other parts of the spectrum is concentrated on such solar cells, the overall efficiency is quite low.
Recently, Weber et al, App. Phys. 15, (1976) 2299-2300 suggested the use of luminescent solar collectors and proposed the use of neodymium doped laser glass or rhodamino-6-G as luminescent material. Their calculations show the relation between trapping of radiation and the refractive index of the collector.
Utilization of solar energy in an efficient manner is one of the main goals of science in order to provide an economic and convenient energy source. One of the main problems encountered is the comparatively high cost of solar cells and of other components of devices designed for solar energy utilization. Large scale terrestrial conversion systems must exceed a certain threshold value of efficiency in order that they be competitive with other available energy sources. Such devices must be installed on a given plot of land, maintained and operated. At present there are known silicon cells which have conversion efficiencies of about 15% at AM=0 and over 18% at AM=1 (AM=air mass through which the solar flux penetrates; AM=1 at sea level for sun overhead). These values are within about 5 to 9 percent of theory. Solar cells are quite expensive and only by concentrating the solar energy on such cells can the problem of cost be overcome, and this only if the concentrating means are inexpensive and efficient.
It has been proposed to link a photovoltaic cell to a small glass plate: See Nature, July 13, 1978, "Planar Solar Energy Converter and Concentrator Based on Uranyl doped Glass." This article set out the basic principles of such energy conversion, but does not describe an efficient embodiment which is of practical value for obtaining the required degree of efficiency.
In U.S. Pat. No. 4,110,123, issued Aug. 29, 1978 there is described an apparatus for converting solar energy into electrical energy, wherein light is collected in a light concentrator comprising a transparent layer, the refraction coefficient of which is greater than that of the ambient medium and which contains fluorescent centers and is fed to a solar cell, characterized in that more than one concentrator/solar cell combination is stacked on top of another through the intermediary of a medium having a smaller refraction coefficient than that of the concentrators, each concentrator being adapted to convert a portion of the incident spectrum into fluorescent light and supply it to a solar cell.
The U.S. patent suggests a similar principle, but it is based on the use of glass cells containing solvents with fluorescent centers. The use of liquid mixtures is complicated and inefficient and such cells require constant maintenance and upkeep. The said patent does not contain any teaching of a working embodiment and thus it constitutes only a mere suggestion of the principle of a multi-layer collector. The present invention discloses specific doped glasses and an arrangement whereby a considerable proportion of the incident energy is converted to utilizable electric energy.